The invention is related to a sonic level measuring technology, and particularly, to a sonic level measuring method of measuring a water level in a high accuracy using a sonic wave in a reservoir, a river, underground water or subterranean, etc. a water level range of which is highly changed.
A changing range of a water level in an artificial reservoir is a few tens of meters. There are many rivers that have a water level changing range of 10 m. It is requested for the hydrology observation so that an absolute error of a water level measurement is below xc2x15xcx9c10 m independent of a water level changing range. An original point of a level meter is often placed at a higher position over a water surface according to the mounting condition of the level meter. In this case, even through the changing range of the water level is small, the water measuring range may become greater. In this case, a distance from the original point of the water level to a water surface is normally 10 to 20 m. If the water level change is the range of xc2x15 m, the water level measuring range becomes 10 to 15 m or 15 to 25 m. In case that a level meter is mounted on a dam in a reservoir, a water level measuring range normally becomes 40 to 80 m. Further, when it is intended to measure a level of the underground water, even though the underground water level is not changed at a high range, the water level is measured by the reference of an original point constituted as a top of an underground water observing tube, which is near the ground surface. A case that the water level measuring range is greater often happened.
There were sonic level meters developed to measure a water level, accurately, if the water level measuring range is great. Typical some sonic level measuring method and apparatus having the relatively higher accuracy are disclosed in patents as follows:
U.S. Pat. No. 5,842,374 published on Dec. 1, 1998
Germany Patent No. 19511234 published on Sep. 11, 1997
Japanese Patent No. 2,756,647 published on Mar. 13, 1998
Korean Patent No. 150714 published on Jun. 16, 1998
These patents are commonly entitled as Measuring method of wide range level and apparatus thereof.
A conventional sonic level measuring method previously disclosed is illustrated in FIG. 1. 1 is a sonic generator, 2 is a waveguide tube and 51,52,53, . . . 5n, 5n+1 are sonic receivers that are arranged in a constant interval l along the waveguide tube 2. The position of the sonic receiver 51 is an original point or zero point to measure the water level. A distance Lx from the original point to a water surface is measured as follows: as the sonic generator 1 generates sonic pulses, the sonic pulse is transited or propagated toward the water surface, reflected on the water surface and then transited upward. At the moment that the sonic pulses reach the original point, the sonic receiver 51 generates an outputting signal. Similarly, as the sonic pulses are advanced, the sonic receiver 5n nearest to the water surface generates the outputting signal and also receives reflected sonic pulses. Therefore, the water level Lx is subject to being measured using four signals received like this. A time interval t1 between time points that the sonic receiver 51 receives the advancing pulse and the reflected pulse is as follows:                               t          1                =                              2            ⁢            Lx                                c            1                                              (        1        )            
A time interval t2 between time points that the sonic receiver 51 and 5n receive the advancing pulse, respectively, is as follows:                               t          2                =                                            L              0                                      C              2                                =                                                    (                                  n                  -                  1                                )                            ⁢                              xe2x80x83                            ⁢              l                                      C              2                                                          (        2        )            
Wherein, L0=(nxe2x88x921) l is a distance that is accurately measured, previously, L0=const, C1 is a sound velocity in the interval Lx, C2 is a sound velocity in the interval L0, and n is the number of the sonic receivers.
A value of Lx to be measured in the expressions (1) and (2) as follows:                               L          x                =                                            t              1                                      2              ⁢                              t                2                                              xc3x97                      L            0                    xc3x97                                    C              2                                      C              1                                                          (        3        )            
Wherein, L0 is a previously known value, t1 and t2 and measured and substituted into the expression (3), and C1 and C2 are not known. Assuming that Lx is approximately equal to L0, and C1≈C2, Lxe2x80x2x is as follows:                               L          x          xe2x80x2                =                                            t              1                                      2              ⁢                              t                2                                              xc3x97                      L            0                                              (        4        )            
In case that C1xe2x89xa0C2, Lxxe2x89xa0L0. A measuring error xcex4Lx of Lx occurs as follows:                               δ          x                =                                                            L                x                xe2x80x2                                            L                x                                      -            1                    =                                                    C                2                                            C                1                                      -            1                                              (        5        )            
When Lx is measured, it is assumed that each of the sound velocity C1 and C2 is changed in the interval""s Lx and L0 as follows:
C1=C0+xcex1({overscore (T)}Lx) 
xe2x80x83C2=C0+xcex1({overscore (T)}LG) xe2x80x83xe2x80x83(6)
Wherein, xcex1 is a temperature coefficient of a sound velocity in air, xcex1≈0.6. C0 is a sound velocity, when an air temperature is zero.
In order to evaluate the error xcex4Lx in the patents described above, assuming that the air temperature in the waveguide tube from the original point 0 to the water surface is changed in a straight gradient of             T      0        -          T      Lx        Lx
as shown in FIG. 2, when C1 and C2 are calculated and then the results are substituted into the error expression (5), the error xcex4Lxe2x80x2x is as follows:       δ          L      x      xe2x80x2        =                    0.5        ⁢                  xe2x80x83                ⁢                  a          ⁡                      (                                          T                0                            -                              T                                  L                  x                                                      )                                                C          0                +                  0.5          ⁢                      xe2x80x83                    ⁢                      a            ⁡                          (                                                T                  0                                -                                  T                                      L                    x                                                              )                                            xc3x97                  Δ        ⁢                  xe2x80x83                ⁢        L                    L        x            
Wherein, T0 is a temperature at the original point and TLx is a temperature at the water surface.
A maximum error xcex4Lxmax appears when xcex94Lmax≈l.                                           Δ            ⁢                          xe2x80x83                                            Lx            ⁢                          xe2x80x83                        ⁢            max                          =                                            0.5              ⁢              a              ⁢                              xe2x80x83                            ⁢                              (                                                      T                    0                                    -                                      T                                          L                      x                                                                      )                                                                    C                0                            +                              0.5                ⁢                                  a                  ⁡                                      (                                                                  T                        0                                            -                                              T                                                  L                          x                                                                                      )                                                                                xc3x97          l                                    (        7        )            
An absolute error xcex94lmax is as follows:                               δ                      Lx            ⁢                          xe2x80x83                        ⁢            max                          =                                            0.5              ⁢              a              ⁢                              xe2x80x83                            ⁢                              (                                                      T                    0                                    -                                      T                                          L                      x                                                                      )                                                                    C                0                            +                              0.5                ⁢                a                ⁢                                  xe2x80x83                                ⁢                                  (                                                            T                      0                                        -                                          T                                              L                        x                                                                              )                                                              xc3x97                      l                          L              x                                                          (        8        )            
If a water level measuring allowance absolute error xcex94xe2x80x2Lxe2x80x2x is given, an interval l between the sonic receivers 5i and 5i+1 is obtained from the expression (8). Assuming that C0=331.6 m and xcex1=0.6, the value of l is as follows:                     l        =                              Δ                          L              x                        xe2x80x2                    xc3x97                                    331.6              +                              0.3                ⁢                                  xe2x80x83                                ⁢                                  (                                                            T                      0                                        +                                          T                                              L                        x                                                                              )                                                                    0.3              ⁢                              xe2x80x83                            ⁢                              (                                                      T                    0                                    -                                      T                                          L                      x                                                                      )                                                                        (        9        )            
Considering that T0=40xc2x0 C., TLx=25xc2x0 C. in summer, and T0=0xc2x0 C., TLx=15xc2x0 C. in winter, in order that xcex94Lx=0.01 m (1 cm), l is as follow:
l=0.78 m in summer
l=0.74 m in winter
If the interval l between sonic receivers is secured to get smaller, the water level measuring absolute error becomes small more and more. Therefore, the conventional sonic level measuring method has great advantages in that the water level absolute error xcex94Lxe2x80x2x is equal throughout a full range to measure the water level independent of the water level measuring range and can be secured to be smaller.
The sonic level measuring method has another method as follows: it saves the mounting cost by which the waveguide tube can be mounted along a gradient surface of a river bank and a reservoir bank unlike other sonic level meters. In this case, a length of the waveguide tube is the multiplication of a value Lx measured by the sonic level meter and sin=45xc2x0, and a water level changing range of a reservoir is 50 m, the length of the wave guide tube must be over 70.7 m by 50 m/sin45xc2x0.
But, the conventional sonic level measuring method has problems as follows: in case that T0 and TLx in the expression (9) are often changed, and the absolute allowance error xcex94xe2x80x2Lxe2x80x2x=xc2x10.001 m, l=0.74-0.78 m must be secured. If it is necessary to measure the water level of a reservoir, more accurately, l=0.37-0.39 m must be secured, so that xcex94xe2x80x2Lxe2x80x2x=xc2x10.0005 m. In this case, if the maximum water level measuring range is 70 m, the number of the sonic receivers is as follows:       n    ≈                  70        ⁢                  xe2x80x83                ⁢        m                    0.37        ⁢                  xe2x80x83                ⁢        m              =      189    ≈    190  
Even if xcex94xe2x80x2Lxe2x80x2x=xc2x10.01 m is secured, n≈85 is required. In case that a large number of the sonic receivers are mounted along the lengthwise portion of the waveguide tube, a water level measuring apparatus becomes complex, and also the failure possibility of the sonic receivers may be heightened. Furthermore, another problem happens as follows: a time interval t1=2 Lx/C1 between an advancing wave and a reflected wave must be measured [referring to the expression (1)]. As Lx becomes longer, the sonic pulse is largely reduced during being transited through the distance 2 Lx. For it, a relatively stronger sonic pulse should be emitted, so a reverberation time becomes longer thereby to increase a noise level. In order to prevent these phenomena, the intensity of the sonic pulse should be adjusted based on the change of the measuring value Lx, but it makes a level meter get complex. An object of the invention is to provide a sonic level measuring method for increasing a water level measuring range by approximate two times as long as a prior art, by which even through an interval l between sonic receivers is selected to be far longer than that in the prior art, a water level measuring error is not increased, and receiving the sonic pulse that is reflected on a water surface and then returned to an original point is not required.
According to the invention, a sonic level measuring method comprises steps of disposing a plurality of N sonic receivers at a constant interval l toward a water surface along the longitudinal portion of a waveguide tube, oscillating sonic pulses to detect the number Ni of the sonic receivers and computing an interval Li=(Nixe2x88x921)l between a sonic receiver 5i disposed nearest the water surface and first sonic receiver 51 that is positioned on the original point for the water level measurement, measuring a transit time       t    1    =      l    C  
that it takes for the sonic pulse to be transited between a sonic receiver 5i-1 and the sonic receiver 5i, measuring a transit time       t    2    =            2      ⁢      Δ      ⁢              xe2x80x83            ⁢      L        C  
from the receiving moment of the advancing sonic pulse until the sonic pulses are reflected on the water surface and then again received by the sonic receiver 5i, computing an interval xcex94L between the sonic receiver 5i and the water surface, adding the interval xcex94L to L1 and obtaining a distance Lx=Li+xcex94L thereby to measure the water level.